Electrolytic silver-plating solution and method



United States Patent M 3,507,758 ELECTROLYTIC SILVER-PLATING SOLUTION AND METHOD Toshiya Oda and Kazutami Hayashi, Tokyo, Japan, as-

signors to Nippon Electric Company, Limited, Tokyo, Japan, organized under the laws of Japan No Drawing. Filed Apr. 12, 1968, Ser. No. 721,091 Claims priority, application Japan, Apr. 17, 1967, 42/24,367 Int. Cl. C2311 /26 US. Cl. 204-46 2 Claims ABSTRACT OF THE DISCLOSURE A silver electroplating solution is provided comprising 20 to 30 g./l. of silver pyrophosphate, 200 to 300 g./l. of potassium pyrophosphate, to 30 g./l. of potassium citrate, 15 to 30 g./l. of potassium carbonate, and 15 to 40 cc./l. of concentrated ammonia water.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an electrolytic silver-plating solution for use in electroplating pure silver over the surface of a metal or similar conductive substances.

In the conventional electrolytic silver-plating methods, the solutions have inevitably been limited to those of the cyanide type because of the stability of the solution, the electric current densities at cathode and anode, the decomposition voltage, the rate of the deposition of silver, and the appearance of the deposited silver. It is true that silver cyanide plating solution excels in efiiciency as mentioned above. The pronounced efliciency of the cyanide type plating solution is due to the presence of cyanogen (CN') ions in the solution. On the other hand, however, the cyanogen ions are converted at the plating process into a cyanogen gas, which is known to be toxic. Therefore, special care and precautions are required in handling such solutions. Thus, notwithstanding its pronounced efliciency, the silver cyanide plating solution is not a desirable working solution.

An object of this invention is therefore to provide a plating solution which will readily deposit a uniform and durable pure silver film over the surface of metal or conductive substance in a short time, and moreover provide high operating safety and long life.

According to this invention it is possible to provide an electrolytic silver plating solution comprising: silver pyrophosphate (Ag P O used as the silver-salt which is the main component therein; ammonia water capable of dissolving the silver salt in it; potassium pyrophosphate (K P O for improving the anode dissolution and also increasing electric conductivity; potassium carbonate (K CO for additionally increasing electric conductivity; and potassium citrate as an agent for producing buffer action to maintain the stability of the solution. As will be mentioned later, the respective contents of the constituents in the solution of this invention are: -30 g./l. silver pyrophosphate; 200-300 g./l. potassium pyrophosphate; 15-30 g./l. potassium citrate; 15-30 g./l. potassium carbonate; and 1540 cc./l. ammonia water.

A characteristic feature of this invention is that an excess of concentrated ammonia water is used to dissolve silver pyrophosphate and an increase in the pH caused by the addition of ammonia can be adjusted easily by citric acid. The silver pyrophosphate to be used in this invention difiers considerably from known metal pyrophosphates and is nearly insoluble in potassium pyrophosphate. For this reason, concentrated ammonia Water is employed to dissolve silver pyrophosphate in the form of an ammonia complex salt. Excess amount of concen- 3,507,758 Patented Apr. 21, 1970 trated ammonia water, however, raises the pH value of the solution to the alkaline region and this often deteriorates the workability. To solve such difiiculty, citric acid is used to lower the pH to 9-11; this restores the workability and furnishes a uniform, lustrous, and fine silverplated surface.

The plating solution of this invention has distinct advantages over conventional ones. First of all, it has an extremely long life since it does not lead to the formation of excessive carbonate which is another defect of the cyanide type plating solution. Secondly, it provides better working environment and lowers cost in installation of plating equipment because no toxic cyanides are used. Up to date, various compounds such as silver nitrate (AgNO silver sulfate (Ag SO silver iodide (Agl) and silver chloride (AgCl) have been tentatively employed as a source of the silver salt in preparing plating solutions, in place of silver cyanide (AgCN) which is the main component of the conventional silver-plating solutions. There is no report, however, that any satisfactory results were obtained. The reason for this is that the instability of the plating solution or the silver salt itself, whose complex ions have much lower stability than that of silver cyanide (Ag(CN) In the solution of this invention, silver pyrophosphate is dissolved by ammonia water, and the stability of the solution is maintained by excessive potassium pyrophosphate and free pyrophosphate ions (P O Silver pyrophosphate is much more stable than other silver salts, and when deposited, it forms a smooth pure silver film. The amount of silver pyrophosphate to be added is 20-30 g./l. because its lower concentration renders it substantially impossible to deposit silver of a desirable thickness in a short time. Also, it would not be economical to increase the silver content above 30 g./l., because further beneficial effects are not expected and because it becomes diflicult to obtain a lustrous fine silver deposit film due to anodic dissolution which considerably changes the concentration of the plating solution.

The amount of potassium pyrophosphate is set at 200- 300 g./l. for reasons that when lower than 200 g./l., the anodic dissolution is not satisfactory, the electric conductivity of the solution is too low to perform a satis factory plating operation, and the solution has poor stability and short life. With the concentration higher than 300 g./l., on the other hand, the plating solution becomes saturated and any further addition does not improve the anodic dissolution, the electric conductivity, stability, and life of the solution. The reason for having set the amount of potassium citrate in the range of 1530 g./l. is that potassium citrate at a concentration lower than 15 g./l. does not serve as an ion-complexing agent or an additive nor maintain the stability of the solution nor does it maintain a sufficiently long life. At a concentration higher than 30 g./l., it does not improve its effectiveness, but rather exerts adverse effects on the solution. The concentration of potassium carbonate was determined within the range of 15-30 g./l., because, at a concentration lower than such value, the electric conductivity of the solution is not sufficient but rather causes adverse effects thereon. More particularly, such a low concentration often causes the silver-plated surface to lose its luster and adhesion to the susbtrate metal and decreases the life of the solution. On the other hand, with its concentration higher than 30 g./l., it does not improve the electric conductivity but rather causes a gleterioration in the smoothness of the deposited silver One of the features of this invention is that the silver is dissolved by ammonia water which is added until a clear solution is obtained. A total of 15-40 cc./l. of ammonia water is generally required, and the pH value raised by the addition of ammonia is adjusted to 9-11 by the addition of citric acid as mentioned above. As is apparent from the preceding explanation, the electrolytic silver-plating solution of this invention consisting of 20-30 g./l. silver pyrophosphate, 200-300 g./l. potassium pyrophosphate, 15-30 g./l. potassium citrate, 15-30 g./l. potassium carbonate, and 15-40 cc./l. ammonia water finds wide application, because it has high chemical stability and electric conductivity. Also, it is economical and ensures stable silver-plated film having uniform thickness.

A better understanding of the objects and features of this invention will be obtained with reference to the following examples.

EXAMPLE 1 A brass sheet with the thickness of 0.45 mm. was cut into a piece of 10 x 6.5 cm. The piece was cleaned by heating in a degreasing solution consisting mainly of sodium orthosilicate, and then washed for a few seconds by dipping in an acid mixture composed of 86 parts concentrated sulfuric acid, 38 parts concentrated nitric acid, 4 parts concentrated hydrochloric acid, and 72 parts pure water. The brass piece was then washed by water for 3 minutes and plated in a Hull cell tester using the following plating solution under the conditions to be shown later.

Silver pyrophosphate, g./l. 20 Potassium pyrophosphate, g./l. 300

Potassium citrate, g./l. 15 Potassium carbonate, g./l. 30 Ammonia water (ammonium hydroxide), cc./l. 35

. Since the pH of the resultant solution went far into the alkaline region, it was adjusted to 11 by adding a 30% aqueous solution of citric acid. The brass piece is then disposed in the plating solution place in the Hull cell tester maintained at the room temperature. The electric current of 1 ampere (a.) was applied for minutes, while stirring the solution with air. The thickness of the uniformly deposited silver film ranged from 3.5 to 4.5,. It is found that the present plating solution can endure long repeated use for electroplating. The current density for obtaining the most satisfactory surface was found to be between 0.3 and 1.5 a./dm. Furthermore, the present solution is not substantially hazardous and is easy to treat as compared with the conventional cyanide type solution. Any toxic gas or characteristic odor does not emanate from the solution. Furthermore, the prolonged plating (more than 1 hour) makes possi ble the formation of a thick coat (above 30,1), because the article to be plated is not subjected to damage by the solution.

EXAMPLE 2 A brass plate of the same dimension as in Example 1 and a Kovar plate of 0.16 mm. thick and x 6.5 cm. wide were cleaned by degreasing as in Example 1. The Kovar plate was dipped into the solution composed of one part concentrated hydrochloric acid and same part pure water at 75-85 C. for 15 seconds. On the other hand, the brass plate was washed with acid under the same conditions as in Example 1. After the washing acid, these plates were respectively washed with water for 3 minutes, and plated in a Hull cell tester using the plating solution of the following composition under the conditions given later.

Silver pyrophosphate, g./l. 20 Potassium pyrophosphate, g./l. 200

Potassium citrate, g./l. 15 Potassium carbonate, g./l. 15 Ammonia water (ammonium hydroxide), cc./l. 25

The pH of the solution was adjusted to 10.5 by pure citric acid solution, and the temperature was kept at room temperature. When the current was caused to flow for 5 minutes at 1 ampere, the thickness of the silver coat was obtained ranging from 4.8-5.5;t, and in both cases a lustrous fine plated surface was obtained.

EXAMPLE 3 A Dumet wire of 32 mm. in length and 0.5 mm. in diameter was inserted at its one end to a hollow cylindrical miniature glass tube of 2 mm. in length, and 0.55 mm. and 1.4 mm. in inner and outer diameters, respectively. Then, it is heated so that the glass tube may be melted to eventually form a glass bead rigidly fixed to the wire.

The wire with the glass bead is then washed in a 10% aqueous solution of nitric acid, and dipped in the present plating solution for 30 minutes with the current density kept at 0.8 a./drn.

The resultant plated silver coating was about 25 microns thick and quite uniform. Even at the bead portion, no corrosion was found, which was not inevitable in the case of the conventional plating solution of cyanide type.

What is claimed is:

.1. Anaqueous alkaline silver electroplating solution consisting essentially of 20 to 30 g./l. of silver pyrophosphate, 200 to 300 g./l. of potassium pyrophosphate, 15 to 30 g./l. of potassium citrate, 15 to 30 g./l. of potassium carbonate, and 15 to 40 cc./1. of concentrated ammonia water.

2. A method of producing a smooth adherent coating of silver on a metal substrate which comprises electroplating silver on said metal substrate from an aqueous alkaline silver electroplating solution consisting essentially of 20 to 30 g./l. of silver pyrophosphate, 200 to 300 g./l. of potassium pyrophosphate, 15 to 30 g./l. of potassium citrate, 15 to 30 g./l. of potassium carbonate, and 15 to 40 cc./l. of concentrated ammonia water.

References Cited UNITED STATES PATENTS 2,504,272 4/1950 McCoy 20446 3,039,942 6/1962 Cox et a1. 204-46 XR 3,039,943 6/1962 Cox et a1. 204-46 XR 3,427,232 2/1969 Natwick 204-46 XR JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner 

